[R] Object equality for S4 objects

[Martin Morgan]

Hi Stavros --

Stavros Macrakis <macrakis at alum.mit.edu> writes:

> To test two environments for object equality (Lisp EQ), I can use 'identity':
>
>> e1 <- environment(local(function()x))
>> e2 <- environment(local(function()x))
>> identical(e1,e2)                      # compares object identity
> [1] FALSE
>> identical(as.list(e1),as.list(e2))    # compares values as name->value mapping
> [1] TRUE                                # (is there a better way to do this?)
>
> What is the corresponding function for testing whether two S4 objects
> are the same object?  It appears that 'identity' for S4 objects
> compares the *value*, not the *object identity*:

Hi Stavros

S4 objects do not have the semantics of environments, but of lists (or
of most other R objects), so it is as meaningful to ask why
identical(s1, s2) returns TRUE as it is to ask why
identical(list(x=1), list(x=1)) returns TRUE.

Martin

>> setClass("simple",representation(a="logical"))
> [1] "simple"
>> s1 <- new("simple"); s2 <- new("simple")
>> identical(s1,s1)
> [1] TRUE                               # not surprising
>> identical(s1,s2)
> [1] TRUE                               # ? not comparing object identity
>> s1 at a <- TRUE
>> s2 at a <- TRUE
>> identical(s1,s2)
> [1] TRUE
>> s1 at a <- TRUE
>> s2 at a <- FALSE
>> identical(s1,s2)
> [1] FALSE
>
> Thanks,
>
>               -s
>
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-- 
Martin Morgan
Computational Biology / Fred Hutchinson Cancer Research Center
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